Developments of nanotechnology call for scalable top-down nanomanufacturing methods to carry on the trends of ever-decreasing critical dimensions and ever-increasing design complexities. Optical lithography, the process of transferring geometric shapes on a mask to a wafer, has been the critical enabling step for determining nanotechnology device performances such as the transistor density and speed in microprocessors. Driven by the fundamental light diffraction and Moore's law, optical lithography researchers have been continuously scaling down the working wavelength in order to produce finer features, leading to dramatically increasing process costs. Current deep-UV tools cost $50M each, and the costs for masks far outweigh those for tools. The cost of next generation extreme-UV tools is expected to exceed $200M each. The future tools will soon become too costly for both industrial productions and scientific researches. Optical lithography is unable meet the long-term demand because of the insurmountable technical barriers and prohibitive costs to further scale down the working wavelength in optical lithography, threatening the nanomanufacturing foundation for future technology growth.
Maskless lithography can write finer features by rastering a nanometer-size beam or probe to generate surface patterns, and has been applied to niche applications such as device prototyping and low-volume production. Among all maskless methods, the electron-beam lithography can provide high resolution beyond the 10-year industry roadmap. Electron-beam lithography also has the highest scanning speed, where a single electron beam is equivalent to the scanning-probe lithography employing 104-106 parallel probes. A 5-6 orders of magnitude enhancement in electron-beam lithography throughput by using millions of high-brightness parallel electron beamlets will take over optical lithography and bring a paradigm shift in nanomanufacturing because of its low tool and process cost, short cycle time, and supreme flexibility.
Current researchers have focused on developing massively-parallel electron-beam lithography and achieved 2-3 orders of magnitude throughput enhancement using variety of methods, but the roadblock has been the lack of an enabling technology to generate millions of high-quality electron beamlets with satisfactory brightness and uniformity. Mapper lithography and reflective electron-beam lithography methods split one electron source into a total number of up to thirteen thousand and one million beamlets respectively. Their throughputs are limited by the total usable beam current, because the Pauli Exclusion Principle places a fundamental limit on the maximum brightness of an electron source. For producing 45-nm features, this source-splitting method has a 10-100 times lower throughput than that of optical lithography. For producing finer features, this method will yield even lower throughputs and become impractical for sub-10 nm features. Others have attempted to develop tip-based field emitter arrays which can break the limit of total useable beam current. However, field emission current changes rapidly with tip sharpness and extraction gap, which leads to tremendous technical barriers to make the emitter array with high yield and to address and control millions of tips in parallel. There are still no practical processes to scale up field emitters into large, high-yield arrays to generate high-quality electron beamlets. These barriers have blocked major efforts to develop field emission display and IBM's “Millipede”. Therefore, improvements are needed in the field.